Why would this not work in spoiler BB Code?

[BirdOPrey5]

I think this is more related to programming then a general issue so it's here, feel free to move it if this is the wrong forum.

There are a number of 'spoiler' bb codes out there that generally hide text from being seen unless a user clicks on a button or does some other action to view the text. However this text is usually still visible if it's the first thing in the thread if someone uses the 'mouse over' preview of the thread. A way to block this is to put text inside square brackets-
example: [this text would not be shown in the mouse-over preview]
So I figured this was a simple enough fix, in the HTML replacement box simply enclose the {param} in square brackets- [{param}] and it would hide the text from mouse over's too, right?

No dice.

When the square brackets are added by the HTML replacement box they show up in the mouse-over as regular text. If I type them into a post directly though they do hide the text. I viewed source of the pages and they are identical... they both show [my sample text] in the HTML code, but vBulletin somehow 'knows' the difference between me typing them in the post and the bbcode putting it in... OK well I just figured it out - on the mouse-over no bbcode is being rendered so the square brackets don't exist yet in the text. oh well... I typed all this out, seems like a shame not to submit this post anyway.